ISLAMABAD—The brazen assassination Tuesday of a popular and progressive Pakistani governor allied with the nation’s president threw an already teetering U.S.-backed government into even greater turmoil.

Salman Taseer, the governor of Punjab province and an avowed opponent of religious extremism, was shot to death at an open-air shopping centre that is frequented by foreigners and wealthy Pakistanis. The gunman was a member of the governor’s own elite police security contingent, officials said.

They said the gunman’s motive was anger at the governor’s call for a pardon of Asia Bibi, a Pakistani Christian woman facing the death penalty for allegedly defaming Islam. The case has divided Pakistan and drawn international criticism of the country’s blasphemy laws.

Taseer’s assassination is a further blow to the ruling Peoples’ Party, which in recent days has been struggling to cope with the defection of a major coalition partner and keep the government afloat. Many of its mourning senior members likened Taseer, a popular and charismatic figure who commanded a large following on Twitter, to Zardari’s late wife, Benazir Bhutto. She too ran afoul of religious extremists and was assassinated three years ago.

Pakistan is a crucial if problematic partner of the United States in the fight against the Taliban, and its latest woes could have repercussions for the war in Afghanistan. President Asif Ali Zardari has been under heavy U.S. pressure to act decisively against insurgents who use tribal borderlands as a sanctuary and a staging ground for attacks on U.S. and other Western forces in Afghanistan.

But Zardari’s government has been weakened domestically by a host of other problems: enduring unhappiness over its handling of last year’s devastating floods, a stumbling economy and a virulent homegrown insurgency.

Interior Minister Rehman Malik said Taseer’s killer, identified as Mumtaz Husain Qadri, had confessed to the shooting and told police he was motivated by the governor’s outspoken opposition to Pakistan’s draconian blasphemy laws, which are strongly backed by Islamist parties.

Like many members of Pakistan’s elite, the 64-year-old Taseer was fluent in English and liberal in his outlook. He was a media mogul who published a newspaper and owned a TV channel.

His killing highlighted long-standing fears about Islamist loyalties within the ranks of Pakistan’s security forces. A witness who did not want to give his name said the gunman, in a police uniform, fired a volley of bullets at the governor.

“I threw myself to the ground when I heard the firing,” the witness said. In the aftermath, blood was pooled on the parking lot of the open-air mall, a popular gathering spot which has a bookstore and newsstand with a large English-language selection and a cafe that serves fare such as quiches, salads and fresh juices.

“He has admitted he is behind the murder,” Malik said of the suspect. “He says the governor had described the blasphemy laws as ‘black laws.’ ” The interior minister said it was not immediately clear whether the killer had acted alone.

The blasphemy laws have been under particular scrutiny since Bibi was condemned to death last year. The case prompted an outcry by international human rights groups, and Taseer had called for a pardon for the Punjabi woman, whose case is still in the courts. That stance made him the target of threats and protests by Islamist groups.

In a Twitter posting on Jan. 1, Taseer wrote, “I was under huge pressure sure 2 cow down b4 rightest pressure on blasphemy. Refused. Even if I’m the last man standing.”

But in a sign of the deep divide over the issue the Pakistani political commentator Murtaza Razvi wrote of overhearing two security guards outside the scene.

“It was rather uncanny to overhear a conversation that I did between two security guards outside the building they were deputed to guard, within minutes of the news of Mr. Taseer’s death breaking. One guard congratulated the other on the assassination while the other responded by saying that the killer was indeed a very courageous man, God be praised.

“This is not the country that makes one feel very safe,” Razvi wrote on his blog.

A tearful party spokeswoman, Fauzia Wahab, called Taseer a “brave man . . . one of a long list of martyrs in our party.” Zardari, through a spokesman, praised Taseer’s “composure, resilience and courage.” Prime Minister Yousaf Raza Gilani announced a three-day period of national mourning.

Pakistan’s latest political crisis erupted Sunday, when the Muttahida Qaumi Movement, or MQM, announced it was quitting the government and going over to the opposition. Since then, Zardari and his top associates have been courting the main opposition party, the Pakistan Muslim League-N, but so far without success.

The Peoples’ Party declared its own two-week mourning period for Taseer, which appeared likely to put the political dispute on hold for the time being. The killing also appeared likely to derail meetings planned Wednesday between top Pakistani officials and a senior Afghan delegation, including President Hamid Karzai, to discuss efforts to bring the Taliban to the bargaining table.